Variation in human skin color is mediated by changes in the activity of epidermal melanocytes, while hair color is determined by the action of melanocytes located in the hair follicle. To date, few genes and pathways have been identified that control a melanocytes decision to adopt an inter-follicular or an extra-follicular fate. Understanding the origins of these two melanocyte subpopulations, and identifying molecules and pathways that may be involved in melanocyte specification may have implications for the treatment of human conditions that specifically affect each subset of cells. To understand the origins of these two melanocyte subpopulations, two complementary approaches are outlined in this proposal. First, using genetic tools that take advantage of site-specific recombination, the fate of melanocyte precursors in wild type mice will be analyzed. Analysis of mosaic animals will be performed to determine if inter-follicular and extrafollicular melanocytes arise from distinct lineages or from a common precursor pool. Second, our laboratory has initiated a project to study a new class of mouse pigmentation mutants that were identified by their dark skin (Dsk) phenotype. We have determined the genetic map location, homozygous phenotype and histologic determinants of ten dominant Dsk mutants, and have identified one class of Dsk mutants, represented by Dsk3, Dsk4, and Dsk6, that has increased epidermal pigment in non-hairy skin. In subsequent studies, we have found that Dsk3 animals have an increased number of epidermal melanocytes in the tail and the footpad, and a reduced number of follicular melanocytes. Dsk4 and Dsk6 animals, on the other hand, have dark skin based on mechanisms independent of pigment cell number. The expansion of the pigment cell population in Dsk3 adult animals may be due to an increase in the number of melanocyte precursors, aberrant melanocyte migration, an increase in cell proliferation or a decrease in programmed cell death. The mechanism by which Dsk3 causes an increased number of epidermal melanocytes will be investigated. In addition, the gene responsible for the dark skin phenotype in Dsk3 mutants will be identified, and the nature of the mutation explored. In future efforts, the mechanisms that control dark skin in Dsk4 and Dsk6 animals will be investigated. Studies of dark skin mutant mice may provide insight into the mechanisms by which epidermal melanocytes choose to contribute to hair color (an intra-follicular fate) or skin color {and extra-follicular fate). This body of work and the proposed career development plan combine my interests in dermatology, cutaneous biology and genetics, and will serve as a springboard from which I will be able to develop an independent research program in a university setting. The support of the Mentored Clinical Scientist Development Award (K08) and the guidance of Dr. Greg Barsh (mentor for this proposal) will facilitate the transition from a mentored trainee to a junior faculty position.